This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-068736, filed Mar. 12, 2001, the entire contents of which are incorporated herein by reference.
The present invention relates to a magnetic recording medium used in, for example, a hard disc apparatus, particularly, to a perpendicular magnetic recording medium utilizing perpendicularly-oriented magnetization. The present invention also relates to a magnetic recording/reproducing apparatus utilizing perpendicularly-oriented magnetization.
Compared with a longitudinal magnetic recording layer, a perpendicular magnetic recording layer, which is capable of recording information in a linear direction at high density even if the recording layer is thick, exhibits a high resistance to thermal decay. Particularly, a double-layered perpendicular recording medium, in which a soft magnetic layer of high permeability is formed below the recording layer exhibiting a perpendicular magnetic anisotropy, permits ideal perpendicular recording because of the mutual function between the head and the soft magnetic layer and, thus, is indispensable for coping with the higher density recording in the future. However, it is known to the art that, if such a soft magnetic layer is formed in the recording medium, the magnetization of the soft magnetic layer is easily changed during rotation of the disc even if the external magnetic field is weak, leading to a lowering of the recorded signal and noise generation.
In order to improve the change in magnetization of the soft magnetic layer, it is proposed in, for example, Jpn. Pat. Appln KOKOKU Publication No. 03-53686 that an antiferromagnetic layer be formed on the soft magnetic layer. It is also proposed in Jpn. Pat. Appln KOKOKU Publication No. 7-105027 that a permanent magnet layer be formed below the soft magnetic layer. Also, it is taught that the direction of magnetization of the permanent magnet layer should be aligned in a radial direction. It is considered reasonable to understand that, if an anisotropy in the radial direction is imparted to the soft magnetic layer, formed is a mechanism for the magnetization rotation to bring about changes in the magnetization in the circumferential direction of the soft magnetic layer, leading to an improved high frequency response capability. JP ""027 quoted above also discloses a laminate structure of a soft magnetic layer and a permanent magnet layer and teaches that it is possible to use an artificial lattice system such as CoFe/Cu as a bias layer for applying a bias instead of the antiferromagnetic materials layer.
However, such a bias layer is used on the premise that the geomagnetic field is weak. It is taught in Jpn. Pat. Appln KOKAI Publication No. 10-283624 that, in the actual apparatus in which the external magnetic field is increased to reach such a high value as thousands of A/cm, it is necessary to use a soft magnetic layer having a thickness of at least 600 nm in order to suppress reversal of a magnetization the bias layer.
As described above, in order to improve stability to an external magnetic field, and the output efficiency, it was considered advisable in the past to increase the thickness of the soft magnetic layer. If the thickness of the soft magnetic layer is increased, it is certainly possible to suppress the generation of domains in the intermediate circumferential region of the disc. However, since it is impossible to suppress the domain generation in regions where the intensity of the antimagnetic field is increased, such as the outer circumferential region and the region in the vicinity of the inner circumferential region of the disc, the domains are also formed in, for example, the data region, making it difficult to remove spike noise. Also, if the soft magnetic layer is thick, inverse domains are easily generated in the soft magnetic layer, leading to an increase in noise, though it is certainly possible to suppress the inversion of the bias. Under the circumstances, it was impossible in the past to obtain a magnetic recording medium, in which a domain is not generated over the entire data region of the disc, which is stable to an external magnetic field, and which is low in noise generation.
A first object of the present invention is to provide a perpendicular magnetic recording medium whose magnetization is not attenuated by the external magnetic field, whose signal is stable even if recording and reproducing is repeated, and whose noise is low.
A second object of the present invention is to provide a magnetic recording-reproducing apparatus whose magnetization is not attenuated by an external magnetic field, whose signal is stable even if recording and reproducing is repeated, and whose noise is low.
According to a first aspect of the present invention, there is provided a perpendicular magnetic recording medium, comprising a nonmagnetic substrate, a cobalt alloy bias layer formed on the nonmagnetic substrate, a soft magnetic layer formed on the cobalt alloy bias layer and containing iron or cobalt as a main component, and a perpendicular magnetic recording layer, wherein the direction of residual magnetization of the cobalt alloy bias layer faces one direction of its radial direction, and the perpendicular magnetic recording medium satisfies the relationship given below:
Mssoftxc3x97(tsoftxe2x88x9240 nm) greater than Mssoftxc3x9740 nm+Msbiasxc3x97tbias 
where, tbias denotes the thickness of the cobalt alloy bias layer, Msbias denotes the saturation magnetization of the cobalt alloy bias layer, tsoft denotes the thickness of the soft magnetic layer, which falls within a range of between 40 nm and 200 nm, and Mssoft denotes the saturation magnetization of the soft magnetic layer.
According to a second aspect of the present invention, there is provided a magnetic recording-reproducing apparatus, comprising a magnetic recording medium, driving mechanism for supporting and rotating the magnetic recording medium, and a magnetic head for recording information in and reproducing the recorded information from the magnetic recording medium, wherein the magnetic recording medium comprises a nonmagnetic substrate, a cobalt alloy bias layer formed on the nonmagnetic substrate, a soft magnetic layer formed on the cobalt alloy bias layer and containing iron or cobalt as a main component, and a perpendicular magnetic recording layer, wherein the direction of residual magnetization of the cobalt alloy bias layer faces one direction of its radial direction, and the perpendicular magnetic recording medium satisfies the relationship given below:
Mssoftxc3x97(tsoftxe2x88x9240 nm) greater than Mssoftxc3x9740 nm+Msbiasxc3x97tbias 
where, tbias denotes the thickness of the cobalt alloy bias layer, Msbias denotes the saturation magnetization of the cobalt alloy bias layer, tsoft denotes the thickness of the soft magnetic layer, which falls within a range of between 40 nm and 200 nm, and Mssoft denotes the saturation magnetization of the soft magnetic layer.
According to the present invention, the intensity of the bias magnetic field is increased so as to make it possible to prevent an inverse domain from being generated over the entire data region of the disc and to suppress spike noise generation. Also, noise generated from the bias layer is effectively shielded by the soft magnetic layer so as to prevent the magnetization from being attenuated by an external magnetic field. Further, magnetic recording with a low noise can be performed with a stable signal even if recording-reproducing is repeated.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.